FIO08-J. Distinguish between characters or bytes read from a stream and -1

The abstract InputStream.read() and Reader.read() methods are used to read a byte or character from a stream, respectively. The  InputStream.read() method reads a single byte from an input source and returns its value as an int in the range 0 to 255 (0x00-0xff). The Reader.read() method reads a single character, and returns its value as an int in the range 0 to 65,535 (0x0000-0xffff). Both methods return the 32-bit value -1 (0xffffffff) to indicate that the end of the stream has been reached and no data is available. The larger int size is used by both methods to differentiate between the end of stream indicator and the maximum byte (0xff) or character (0xffff) value. The end of stream indicator is an example of an in-band error indicator. In-band error indicators are problematic to work with, and the creation of new in-band-error indicators is discouraged.

Prematurely converting the resulting int to a byte or char before testing for the value -1 makes it impossible to distinguish between characters read and the end of stream indicator. Programs must check for the end of stream before narrowing the return value to a byte or char.

This rule applies to any method that returns the value -1 to indicate the end of a stream. This includes any InputStream or Reader subclass that provides an implementation of the read() method. This rule is a specific instance of rule NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data.

Noncompliant Code Example (byte)

FileInputStream is a subclass of InputStream.  It will return -1 only when the end of the input stream has been reached. This noncompliant code example casts the value returned by the read() method directly to a value of type byte and then compares this value with -1 in an attempt to detect the end of the stream.

FileInputStream in;
// initialize stream 
byte data;
while ((data = (byte) in.read()) != -1) { 
  // ... 
}

If the read() method encounters a 0xFF byte in the file, this value becomes indistinguishable from the -1 value used to indicate the end of stream, because the byte value is promoted and sign-extended to an int before being compared with -1. Consequently, the loop will halt prematurely if a 0xFF byte is read.

Compliant Solution (byte)

Use a variable of type int to capture the return value of the byte input method. When the value returned by read() is not -1, it can be safely cast to type byte. When read() returns 0x000000FF, the comparison will test against 0xFFFFFFFF, which evaluates to false.

FileInputStream in;
// initialize stream 
int inbuff;
byte data;
while ((inbuff = in.read()) != -1) { 
  data = (byte) inbuff;
  // ...  
}

Noncompliant Code Example (char)

FileReader is a subclass of InputStreamReader which is in turn a subclass of Reader.  It also returns -1 only when the end of the stream has been reached.   This noncompliant code example casts the value of type int returned by the read() method directly to a value of type char, which is then compared with -1 in an attempt to detect the end of stream. This conversion leaves the value of data as 0xFFFF (e.g., Character.MAX_VALUE) instead of -1. Consequently, the test for the end of file never evaluates to true.

FileReader in;
// initialize stream 
char data;
while ((data = (char) in.read()) != -1) { 
  // ... 
}

Compliant Solution (char)

Use a variable of type int to capture the return value of the character input method. When the value returned by read() is not -1, it can be safely cast to type char.

FileReader in;
// initialize stream 
int inbuff;
char data;
while ((inbuff = in.read()) != -1) { 
  data = (char) inbuff;
  // ...  
}

Risk Assessment

Historically, using a narrow type to capture the return value of a byte input method has resulted in significant vulnerabilities, including command injection attacks; see CA-1996-22 advisory. Consequently, the severity of this error is high.

Automated Detection

Some static analysis tools can detect violations of this rule.

Related Guidelines

Bibliography